1. Field of the Invention
The present invention relates to a composite metal sheet comprising at least two metal sheets and an intermediate resin layer formed therebetween, which has excellent electric resistance weldability and adhesion strength. This composite metal sheet is useful for a damping sheet, a sound-insulating sheet, a light-weight steel sheet and the like.
2. Description of the Related Art
A composite steel sheet for absorbing vibrations or the like, which comprises at least two steel sheets and a resin layer interposed therebetween, is known, and the present inventors have already proposed composite steel sheets of this type in Japanese Unexamined Patent Publication No. 50-79,920. Also, similar techniques are disclosed in Japanese Unexamined Patent Publications Nos. 53-128,687, 57-24,251, 57-146,649, 57-163,559, 57-163,560 and 58-132,550. None of these composite steel sheets, however, can simultaneously satisfy the electric resistance weldability and adhesion strength requirements.
Japanese Unexamined Patent Publication No. 57-146,649 teaches that an electrically conductive substance such as iron powder, mill scale or carbon graphite powder is incorporated into an intermediate resin layer so that it occupies 10 to 50% by weight of the intermediate resin layer, and that at least 50% by volume of the electrically conductive substance consists of particles having a particle size not smaller than one half the thickness of the intermediate resin. As specific examples of the electrically conductive substance, the above publication discloses carbon graphite and mill scale in which particles having a particle size of 150 to 325 mesh (i.e., about 100 to about 50 .mu.m) occupy at least 50% by volume of the total particles. When the thus-disclosed composite steel sheet is subjected to electric resistance welding under a pressure, the welding current must be increased to a level higher than in the case of ordinary steel sheets. However, the allowable range of the applied electric current is very narrow, and if the electric current exceeds the permissible upper limit to even a slight degree, an electric connection occurs at points other than the welding points whereby sparks are generated and the steel sheet is readily perforated. If the amount of electrically conductive particles is increased to make it possible to perform welding with a small electric current under a low pressure, the adhesion strength is reduced.